In these years, there is an increasing demand for improving the corrosion resistance of building materials for accommodating the acidifying atmospheric environment and construction works on the shore or in the sea. For automotive bodies, on the other hand, corrosion resistance in snow melting salt spreading areas and seaside areas is a problem. One economically advantageous measure for improving corrosion resistance is zinc coating, especially zinc hot dipping or galvanizing. Further heat treatment to convert the zinc coating into a Fe-Zn alloy can improve weldability and corrosion resistance after paint coating. As the problem of global greenhouse effect has drawn great attention, discussions are made on energy savings, especially fuel consumption improvement and body weight reduction of automobiles. One effective approach is to increase the strength of steel strips. Galvanizing or galvannealing of high-strength steel strips is then required in order to meet the above-mentioned demand for corrosion resistance.
Galvanized or zinc hot dipped steel strips are manufactured by means of a continuous galvanizing line (CGL) by continuously carrying out the steps of degreasing by burning off of rolling grease or with alkali, annealing reduction, cooling, molten zinc bath dipping, and coating weight adjustment by gas wiping. Galvannealing or alloying is generally carried out immediately after the wiping step. As is well known in the art, readily workable high-strength steel strips contain Si, Mn, P, etc. as additive components, which tend to concentrate and be oxidized at the steel strip surface, which substantially detracts from wettability to molten zinc, eventually leading to uncoated defects. As a solution to this problem, it was proposed to carry out electroplating of Ni systems (JP-A 262950/1985 and 147865/1986) or electroplating of Fe systems (JP-A 194156/1990) to restrain concentration and oxidation of the additive components at the steel strip surface prior to the entry of steel strip into the CGL.
Electroplating of Ni systems or electroplating of Fe systems prior to the entry of steel strip into the CGL is effective for restraining concentration and oxidation of the additive components at the steel strip surface and thus enables galvanizing of high-strength steel strips containing Si, Mn, P, etc., but with the accompanying problems of more complex process, higher cost, and lower productivity due to the installation of an additional electroplating equipment. It is then desired to develop a method capable of galvanizing high-strength steel strips containing Si, Mn, P, etc. without raising these problems.